There are a number of different types of semiconductor-based imagers, including charge coupled devices (CCDs), photo diode arrays, charge injection devices and hybrid focal plane arrays. CCDs are often employed for image acquisition for small size imaging applications. CCDs are also capable of large formats with small pixel size and they employ low noise charge domain processing techniques. However, CCD imagers have a number of disadvantages. For example, they are susceptible to radiation damage, they exhibit destructive read out over time, they require good light shielding to avoid image smear and they have a high power dissipation for large arrays.
Because of the inherent limitations in CCD technology, there is an interest in complementary metal oxide semiconductor (CMOS) imagers for possible use as low cost imaging devices. A fully compatible CMOS sensor technology enabling a higher level of integration of an image array with associated processing circuits would be beneficial to many digital applications such as, for example, in cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems, star trackers, motion detection systems, image stabilization systems and data compression systems for high-definition television.
A CMOS imager circuit includes a focal plane array of pixel cells, each one of the cells including either a photo diode, a photogate or a photoconductor overlying a doped region of a substrate for accumulating photo-generated charge in the underlying portion of the substrate.
In a conventional CMOS imager, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to a floating diffusion node accompanied by charge amplification; (4) resetting the floating diffusion node to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. The charge at the floating diffusion node is typically converted to a pixel output voltage by a source follower output transistor. The photosensitive element of a CMOS imager pixel is typically either a depleted p-n junction photo diode or a field induced depletion region beneath a photogate. For photo diodes, image lag can be eliminated by completely depleting the photo diode upon readout.
CMOS imagers have a number of advantages, including for example low voltage operation and low power consumption. CMOS imagers are also compatible with integrated on-chip electronics (control logic and timing, image processing, and signal conditioning such as A/D conversion); CMOS imagers allow random access to the image data; and CMOS imagers have lower fabrication costs as compared with the conventional CCD since standard CMOS processing techniques can be used. Additionally, low power consumption is achieved for CMOS imagers because only one row of pixels at a time needs to be active during readout and there is no charge transfer (and associated switching) from pixel to pixel during image acquisition. On-chip integration of electronics is particularly advantageous because of the potential to perform many signal conditioning functions in the digital domain (versus analog signal processing) as well as to achieve a reduction in system size and cost.
With the need for enhanced resolution and a higher level of integration of an image array with associated processing circuit using CMOS imaging devices, there is a need for improving the characteristics of CMOS image arrays. As such, it would be beneficial to minimize if not eliminate the loss of light transmitted to pixel arrays during image acquisition. Enhanced light transmission upon pixel arrays is one means to enhance the image processing and imaging capabilities of CMOS chip devices. These enhancements in CMOS imagers have many applications, including use as imagers in color cameras.
Accordingly, there is needed an improved CMOS imaging device capable of receiving and propagating light to photo diodes with minimal loss of light transmission to the photo diodes. There is also needed an improved method for fabricating CMOS imaging devices, in which there is a high level of transmission of light to photo diodes and that reduces the drawbacks in CMOS imaging of the prior art. Methods of fabricating a pixel array exhibiting these improvements are also needed.